The trend of current light emitting diode of visible light is that the intensity of illumination of light emitting diode is more and more stronger, while the volume is more and more compact.
U.S. Pat. Nos. 5,008,718 and 5,233,204 disclosed a light emitting diode with a transparent window layer. By this kind of light emitting diode, the crowding effect occurring in the conventional light emitting diode is reduced, wherein the current spread to emit light from the light emitting diode is increased. As a result, the illumination of the light emitting diode is apparently enhanced.
Moreover, U.S. Pat. No. 5,237,581 and No. 4,570,172 disclosed a light emitting diode having a semiconductor multilayer reflector, namely, DBR (distributed Bragg Reflector). By this light emitting diode, the light transmitting to the substrate is reflected backwards so as to penetrate through the light emitting diode. Accordingly, the light illumination of the light emitting diode is increased.
A cross sectional view of a conventional light emitting diode is illustrated in FIG. 1. The light emitting diode 100 includes a semiconductor substrate 102, a second ohmic contact electrode 101 formed on the rear side of the semiconductor substrate 102, a light generating region 103 formed on the semiconductor substrate 102, and a first ohmic contact electrode 106 formed on the light generating region 103. Because of the current crowding effect, critical angle of the emitting light and light absorption of the substrate, the illumination in this light emitting diode is not suitable. The light generating region 103 is formed by a P type region and an N type region, and then the light generating region 103 is grown on the gallium arsenide substrate 102. Therefore, the crystal lattice constants in most of the light generating region 103 are matched with that of the gallium arsenide substrate. Namely, the light emitting diode of visible light is directly fabricated on the gallium arsenide substrate 102. However, since the energy gap of the gallium arsenide is 1.43 eV which is smaller than that of the visible light and the light emitted from the diode is non-isotropic, part of the light enters the substrate and is absorbed by the gallium arsenide. U.S. Pat. Nos. 5,008,718 and No. 5,233,204 disclosed a transparent window layer structure for increasing the output light of a light emitting diode. Referring to FIG. 2, the structure of the light emitting diode 200 is formed by a transparent window layer 204 is grown on the light emitting diode 100 shown in FIG. 1. The proper material suitable for the transparent window layer 204 includes GaP, GaAsP, and AlGaAs, etc., whose energy gap is larger than those of the materials in the AlGaInP light generating region. Under this condition, the optic critic angle can be increased and the current crowding effect is reduced so as to enhance the illumination of the light emitting diode. However, in the electric property, since the materials on the uppermost layer of the transparent window layer 204 and the AlGaInP light generating region have a hetero junction, the energy gap difference causes the positive foward bias voltage V.sub.f of the light emitting diode to increase. As a result, the power loss of using the light emitting diode is increased.
The U.S. Pat. Nos. 5.237,581 and 4,570,172 disclosed a light emitting diode 300 with a multilayer reflecting structure, as shown in FIG. 3. The structure of FIG. 3 includes a semiconductor substrate 302, a lower multilayer reflector 305 formed on the semiconductor substrate 302, a light generating region 303 formed on the lower multilayer reflector 305, an upper multilayer reflector 304 formed on the light generating region 303, a first ohmic contact electrode 306 on the upper multilayer reflector 304, and a second ohmic contact electrode 301 on the rear side of the semiconductor substrate 302. In this prior art light emitting diode, the lower multilayer reflector 305 serves to reflect 90% of the light emitted from the light generating region to the light absorption substrate, while the upper multilayer reflector serves to guide light to the upper surface of the light emitting diode. Therefore, the problem of light absorption by the substrate is improved, and the problem of bad illumination from enlarging the critical angle is also improved. However, since the multilayer reflector has many hetero junctions, the effect of energy gap difference is enlarged. As a consequence, although the aforesaid DBR structure disclosed in U.S. Pat. Nos. 5.237,581 and 4,570,172 can reflect the light impinging on the substrate by the DBR structure, the DBR has a high reflective index only for normal incident light (shown in D1 of FIG. 3), while for oblique incident light (such as D2, D3, and D4 shown in FIG. 3) the reflective index is very small. Thus it is only a slight improvement to the illumination of a light emitting diode in the visible light band. Whereas the DBR structure increase the cost and difficulty of growing the thin film epitaxial layer. U.S. Pat. No. 5,376,580 disclosed a light emitting diode with wafer bonding, wherein a gallium arsenide substrate serves as a temporary substrate to grow a light emitting diode structure (including a confinement layer, an active layer and another confinement layer). Then the light emitting diode structure is adhered to a transparent substrate, and the GaAs substrate is removed. Therefore, the light absorption by the substrate can be solved completely. Whereas the transparent substrate disclosed in the aforementioned U.S. Pat. No. 5,376,580 is made by GaP which is very expensive and has an orange color. The light from LED to the substrate has a slight color. Further, in high temperature, the GaP as a transparent substrate needs to be processed for a long period of time (about 600.about.700.degree. C. for at least one hour), this results in a bad effect to the p-n junction of LED.